1. Field of the Invention
The present invention relates to semiconductor memory devices. More specifically, the present invention relates to a NAND non-volatile two-bit flash memory.
2. Description of the Related Art
A non-volatile semiconductor memory device can retain information even when the device is left without power for indefinite periods of time. According to its function, non-volatile memory devices can generally be categorized into read only memory (ROM), programmable read only memory (PROM), electrical programmable read only memory (EPROM), electrical erasable programmable read only memory (EEPROM) and EEPROM flash memory. Because of its capability of faster erasure and programming in blocks, the EEPROM flash memory has been widely used for high volume data storage in devices such as memory cards, memory sticks, personal digital assistants (PDA) and MP3 players.
EEPROM flash memories can be roughly classified into NAND-type and NOR-type. A NAND flash memory has a plurality of memory cells serially connected as a string to share sources and drains with the neighboring cells. A NOR flash memory has a plurality of memory cells connected in parallel, each of which has its own source and drain. Due to this difference, a NAND flash memory has a higher cell density than a NOR flash memory.
U.S. Pat. No. 6,885,586 describes a self-aligned split-gate NAND flash memory. Each memory cell has a control gate and a floating gate which are stacked and self-aligned with each other, and a split gate interposed between stacked gates for erasure. The floating gates are fabricated from a conductive material such as polysilicon or amorphous silicon with rounded side edges to improve erasure and programming performance. In the erasure operation, an enhanced electric filed resulted from the rounded curvature of floating gates facilitate electrons to emit from the rounded side edge of the floating gate to the corresponding split gate by Fowler-Nordheim tunneling effect. In the programming operation, electrons are injected from the channel region into the floating gate by hot electron injection or Fowler-Nordheim tunneling effect. A memory cell can store one-bit information.
The demand is increasing for large volume data storage with low cost. One approach to increase the density of data storage is for a memory cell to be capable of storing two-bit information.